It has been observed that 2,5-diethyltoluene (2,5-DET) is an excellent desorbent in certain separations using a simulated moving bed, such as Sorbex.TM. separations technology (see, for example, Handbook of Petroleum Refining Processes, edited by Robert A. Meyers, 8-80 et ff., McGraw-Hill Book Company (1986)). Unfortunately, the material commonly available as 2,5-diethyltoluene is a mixture containing substantial proportions of the 3,5- and 2,6-isomer, and may also contain significant amounts of p-cymene (4-isopropyltoluene). The separation of such a mixture is itself difficult, so that relatively pure 2,5-diethyltoluene is not available in commercial quantities, and any separation performed to provide the latter would be prohibitively expensive. To compound the problems associated with the use of commercially available 2,5-DET, although some of the DET isomers are "neutral", in the sense of neither hindering nor promoting the adsorbent properties of the 2,5-isomer, other isomers, such as 3,5-diethyltoluene, are definitely deleterious and desirably should be present in as low a level as possible when 2,5-diethyltoluene is used as a desorbent.
Although the methylation of 1,4-diethylbenzene would appear to be the logical choice of means for preparing 2,5-diethyltoluene, such a method is rife with difficulties, some foreseen, others unanticipated. A catalyst which promotes alkylation also promotes the migration of alkyl groups on the ring (i.e., intramolecular alkylation), so that the production of isomers where the ethyl groups are not in the para-orientation may be anticipated. A catalyst which promotes alkylation also promotes transalkylation (i.e., intermolecular alkylation) where the ethyl group from one molecule is transferred to another molecule giving one monoethylbenzene molecule and one triethylbenzene molecule. A catalyst which promotes alkylation also promotes dealkylation, i.e., removal of alkyl groups from the ring, thereby affording monoethylbenzene. Finally, catalysts which promote alkylation are generally nonspecific, affording a range of mono- and polyalkylated products.
In view of the foregoing remarks the task of preparing 2,5-diethyltoluene containing a limited and well defined isomer distribution and with minimal polyalkylation and dealkylation products seemed formidable indeed. Consequently we were delighted and rather surprised to find that certain catalysts appeared to effect the methylation of 1,4-diethylbenzene with the desired selectivity. More particularly, when methanol was the alkylating agent in at least, and preferably more than, one molar proportion relative to 1,4-diethylbenzene it was found that quite good selectivity was achieved, rather contrary to the expectation that optimum selectivity would occur at a methanol concentration equivalent to less than one molar proportion.